NHC’s Tropical Cyclone Report: Hurricane Fay

Hurricane Fay at 7:13am October 12th as seen on Bermuda Weather Service Radar as winds were peaking across the island. The Hurricane made landfall about 2 hours prior to this image.

Today, the National Hurricane Center (NHC, based in Florida) released their Tropical Cyclone Report on Hurricane Fay. The post-season report summarizes the meteorological history of the cyclone and goes through an extensive post-analysis process on all available observations from the cyclone. Because wind measurements are highly dependent on elevation and exposure, a part of this process seeks to adjust and convert observed sustained so that they represent a 1-minute sustained wind at 10m above sea level. This makes all the observations comparable with each other and the advisory information. A tropical cyclone report is created for every Tropical Cyclone in the Atlantic and Eastern Pacific at the end of each hurricane season.

NHC meteorologists found that Fay had strengthened to a Category 1 hurricane roughly 2 hours before making landfall at 5:10am October 12th in the eastern parishes of Bermuda with 70kt (80mph) 1-minute sustained winds. While Bermuda has taken many direct-hits from hurricanes and tropical storms, Fay was the first hurricane landfall in Bermuda since Hurricane Emily made landfall on the island in September 1987. Recall that landfall is determined by the center of a tropical cyclone crossing the coast – damaging winds are usually located some distance away from the center, and the strongest winds of a tropical cyclone can impact land without landfall occurring.

As of the 12am (BDA time) advisory on October 12th, aircraft had measured 55kt surface winds, and 79kt flight level (~5,000ft) winds. Those stronger winds were occurring in a region of the storm that was devoid of deep convection and so they were not mixing efficiently to the surface – surface winds were in the 35-45kt range in that area. This led the NHC forecasters to keep Fay as a 60kt Tropical Storm. However, satellite estimates from the University of Wisconsin-Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS) Dvorak Technique (at 3am and 9am local time) and the Advanced Microwave Sounding Unit (AMSU) scans (at 7am) suggested that peak winds were near 75kts and 63-75kts respectively, however, these estimates were coming in as Fay was passing over and moving away from Bermuda where limited reports (mainly of strong tropical storm strength winds) were coming in. Further, the NHC favors the more reliable ground-based and aircraft observations over satellite estimates. Keep in mind that hurricane force winds start at 64kts and there is often little difference between a 60kt tropical storm and a 65kt hurricane.

Observations of winds ranging from violent tropical storm force to hurricane force came in at locations around Bermuda by 7am local time – as Fay was moving away from the island. Peak winds were observed between 6:55am and 8:03am below. Winds were strongest in the western and central parishes, and on the hilltops exposed to northwesterly winds where multiple locations measured gusts over 100mph. These violent winds resulted in isolated roof damage and widespread tree and utility pole damage.

When adjustments for comparability yielded several observations over hurricane force. That when combined with satellite estimates of Fay just before and as Fay made landfall indicate that Fay indeed had hurricane force winds. In fact, winds were likely near 70kts based on this post-storm analysis. It should also be noted that wind gusts can be significantly higher than sustained winds, especially in the deep convection (heavy rain/thunderstorms) of a tropical cyclone. That was the case here for Fay where gusts were on the order of 20-40kts higher than sustained winds.

high (>50%) chance of formation within 120 hours was issued 6 hours before formation

I has been noted that historically, forecasting the formation of tropical cyclones with non-tropical origins is less skillful than it is for those with tropical origins.

48hours of tropical storm watch before the onset of tropical storm conditions, 24hours of tropical storm warning before the onset of tropical storm conditions, and 12 hours of hurricane watch before the onset of hurricane conditions. This is compared to 48hours, 36hours, and 48hours respectively in ideal forecasting conditions.

Rainfall, both of these sites experienced wind damage that impacted these measurements:

Airport (official) – 1.87″

“West End” observer (unofficial) – 3.70″

Storm Tide as measured on the north side of St. George’s island at ESSO pier:

1.78feet (0.54m)

Likely higher is some western parishes in spots where the strongest winds were onshore

Lowest Pressures:

Airport (official) – 985.1mb

Lowest (unofficial ) -984.5mb

Elsewhere between 985 and 987mb

Norwegian Dawn Cruise Liner observed 55kt sustained winds, 994.6mb at roughly 7am October 12th just to the north of Bermuda.

Peak unadjusted winds included in the NHC report from around the island ordered by gust strength, it should be noted that at least some of the winds at the airport were estimated: